Samper was among 15 biological and social scientists who led the U.N. Millennium Ecosystem Assessment. From 2001 to 2005, the study involved the work of more than 1,360 experts worldwide. Their findings provide a state-of-the-art scientific appraisal of the condition and trends in the world's ecosystems and the services they provide, as well as the scientific basis for action to conserve and use them sustainably.

The study found that humans have changed ecosystems worldwide more rapidly and extensively in the last 50 years than in any other period in recent history, largely to meet growing demands for food, fresh water, timber, fiber and fuel. Such changes have significantly contributed to the well-being and economic development of humankind, while becoming increasingly detrimental to many other ecosystem services.